1. The Field of the Invention
The present invention relates to semiconductor devices and methods for their construction. More particularly, the present invention relates to deposition of titanium nitride barrier layers in semiconductor devices. More specifically, the present invention provides titanium nitride barrier layers of low resistivity and high conformity.
2. The Background Art
Titanium nitride barrier layers are formed over silicon dioxide or glass layers such as an ILD (interlayer dielectric) or IMD (intermetal dielectric) which have contact holes or vias etched through them. In the case of an ILD the contact hole contacts a source or drain region in a single crystal silicon or silicide substrate. Some contact holes may also contact a polysilicon gate electrode.
Titanium nitride barrier layers are used for at least three purposes. First, they block diffusion of species between a single crystal silicon substrate and a metal in the contact hole. If significant amounts of metal find their way from the metal contact into the silicon substrate, the device can be destroyed. Second, the titanium nitride facilitates adhesion of tungsten (when that metal is used in a contact hole) during tungsten deposition processes. For this reason, the titanium nitride barrier layer is often referred to as a "glue" layer. Finally, when tungsten hexafluoride (WF.sub.6) is used to deposit tungsten it may chemically attack the silicon substrate, thus damaging the semiconductor device. The titanium nitride barrier layer prevents WF.sub.6 from reaching the single crystal silicon substrate where it could do such damage.
Deposition of titanium nitride typically takes place by a "metal organic chemical vapor deposition" reaction, sometimes referred to as MO-CVD. This process employs an organotitanium compound which thermally decomposes on the substrate to form titanium nitride. Usually ammonia is added to the gaseous reaction mix to act as a reducing agent. In most systems, the organotitanium compound is tetrakis (diethylamido) titanium (TDEAT). In some cases, tetrakis (dimethylamido) titanium (TDMAT) may be employed in its place.
It is important that the titanium nitride barrier layer be deposited conformally and with good step coverage. A conformally deposited layer has nearly equal thickness on the top and bottom horizontal surfaces of a trench as well as the vertical side walls of that trench.
Step coverage is defined as the ratio of the thickness of a deposited film at the bottom horizontal surface of a trench to the thickness of that layer at the top horizontal surface just outside of the trench. Thus, in FIG. 1, for example, the step coverage is given by the ratio of a lower film thickness 2 to an upper film thickness 4. In FIG. 1, a conformal film 6 (e.g., titanium nitride) is formed over a dielectric layer 8, which is in turn formed on a semiconductor substrate 10. Dielectric layer 8 has a contact hole 12 etched therein. Contact hole 12 has as a property an aspect ratio defined as its depth (vertical dimension) divided by its width (the horizontal extent of the opening at the top of the hole).
Insufficient conformal coverage and/or too small step coverage can lead to semiconductor device failure because the titanium nitride barrier layer inadequately serves one of its functions. For contact holes having relatively low aspect ratios and wide trenches, these problems are not significant. However, as device density increases and aspect ratios correspondingly increase, it becomes more difficult to deposit conformal layers with good step coverage. Generally, for MO-CVD of titanium nitride, lower deposition temperatures always provide higher step coverage. Thus, MO-CVD of titanium nitride can provide good step coverage of contact holes of aspect ratios on the order of 5:1 when the deposition temperatures are kept below about 300.degree. C.
Unfortunately, titanium nitride barrier layers deposited at low temperatures suffer from certain problems. Most notably, the resistivity of these materials is too high, typically above about 1200 .mu.Ohm-cm which degrades device performance by slowing signal transmission between devices. Although the mechanism responsible for the resultant high resistivity of titanium nitride barrier layer deposited at low temperature has not been conclusively established observations have been made that such titanium nitride barrier layer is more porous and has a higher oxygen content than barrier layers deposited at higher temperatures.
The chemical vapor deposition (CVD) of titanium nitride is carried out under vacuum conditions with low content of oxygen. However, oxygen and moisture may find their way into the titanium nitride barrier layer after the device is formed and removed from the CVD chamber. Thus, possibly, the porous, low density titanium nitride barrier layers deposited at lower temperatures more tenaciously retain oxygen and moisture than those barrier layers deposited at higher temperatures.
In any event, it has become apparent that as device size shrink and contact holes become increasingly narrow and deep, improved techniques for depositing titanium nitride barrier layers are required. Such improved techniques should provide titanium nitride barrier layers which are conformal, have good step coverage and low resistivity.